Subsequent to the 1861 enclosure of an additional floor behind the parapet, the height of the Foundation Building was six stories above grade, two of which were rented out to shops and offices. With increasing demand for educational space in the building, Petersen was commissioned on several occasions between the years 1878 and 1881 to augment his original intention. In 1878, the north portico was enclosed to create interior space on the fourth and fifth floors.
In 1879, an iron-framed pavilion with clerestory windows was added on the roof, raising the height of a portion of the sixth floor. A seventh story was also added around the south and east sides of the building, flanking the pavilion.
In 1880, a domed room with an exterior clock was constructed above the seventh story, at the center of the south fa├žade. It was flanked by two pavilions on either side, bringing the Foundation Building to a total of eight stories above grade.